American orbital launch vehicle. The Delta launch vehicle was America's longest-lived, most reliable, and lowest-cost space launch vehicle. Delta began as Thor, a crash December 1955 program to produce an intermediate range ballistic missile using existing components, which flew thirteen months after go-ahead. Fifteen months after that, a space launch version flew, using an existing upper stage. The addition of solid rocket boosters allowed the Thor core and Able/Delta upper stages to be stretched. Costs were kept down by using first and second-stage rocket engines surplus to the Apollo program in the 1970's. Continuous introduction of new 'existing' technology over the years resulted in an incredible evolution - the payload into a geosynchronous transfer orbit increasing from 68 kg in 1962 to 3810 kg by 2002. Delta survived innumerable attempts to kill the program and replace it with 'more rationale' alternatives. By 2008 nearly 1,000 boosters had flown over a fifty-year career, and cancellation was again announced.

The Delta space launch family had its roots in the Thor IRBM. Thor was a 2400 km range missile with essentially the same characteristics as the Army's Jupiter. It was developed by Douglas Aircraft in one of the most accelerated crash programs in history. 'Chief Designer' of the Thor was Jack Bromberg. He was not educated formally as an engineer, but was smart and dynamic. Company owner Donald Douglas had hired him back in the 1930's. He was a major influence in Douglas' winning response to the USAF request for proposal, which emanated from USAF Missile Czar General Bernard Schriever.

The Air Force requirement was to build an IRBM around a discarded rocket motor design. The engine had been developed as the sustainer engine for the Atlas missile when the requirement was to launch a 2700 kg thermonuclear warhead over an intercontinental range. When it became apparent that the warhead could be reduced to less than half that weight, the Atlas was downsized, and the 68 metric ton thrust sustainer motor was no longer required.

Douglas flew a first prototype only thirteen months after go-ahead. Sixty Thors were deployed to missile sites in Great Britain under Project Emily. After a few years the Thors were withdrawn from Britain (and the Jupiters from Turkey) as part of the secret codicil of Kennedy's deal to end the Cuban Missile Crisis. A few continued in an offensive military role as a nuclear-tipped anti-satellite system, based on Johnson Atoll in the Pacific, until the end of the early 1970's. Surplus Thors were used for a variety of suborbital re-entry vehicle tests.

After the Sputnik shock, all available assets were pressed into service. Thor was developed into a workhorse for the space program. The Able stage developed for the Vanguard project was mated to the Thor. A fundamental problem was that the thrust to weight ratio of the basic vehicle left little margin for growth or larger upper stages. This was solved by using Castor solid rockets strapped to the base of the vehicle to increase the lift-off thrust. Almost by accident rather than design, Thor developed into the Delta, the standard American medium-lift launcher, continually evolving through the use of more and larger strap-on boosters, a cylindrical and stretched core, and enlarged upper stages. The original Aerojet Able upper stage evolved into the Delta. From 1959-1972 the US Air Force and NASA flew versions of the launch vehicle using the same core and solid booster technology, but with the Agena upper stage rather than the Delta. After going through a bewildering myriad of military versions it emerged as the standardized, reliable and economical Delta commercial satellite launch vehicle by the mid-1970's. Improved Delta versions continued in production into the next millennium, with nearly 1,000 airframes completed, and the end of production announced yet again in 2008.

The innumerable 'deaths' and resurrections of the Delta are described in Aerojet - The Creative Company, 1995:

Using NASA terminology, their first Delta launch vehicle was basically an Air Force Able consisting of a Thor first stage, an Able second stage, and a solid propellant third stage of nominally 2760 lbf thrust produced by Allegheny Ballistics Laboratory (ABL). These vehicles and their powerplants are discussed above as part of the Able family. The first NASA contracts for these upper stages were in 1959, deliveries began in 1961, and numerous additional contracts followed. Also in 1961, NASA shifted to the Ablestar configuration. The first launch using this configuration occurred on May 13, 1960 and, according to one source on NASA history, did not reach orbit because of a malfunction in the Aerojet stage. Aerojet's findings indicate that it never got the signal to fire. The second attempt on August 12, 1960 was successful, and these engines were used until years later when the more powerful Air Force Delta second stage ablative engines became available.

At this point the Air Force, realizing the need for a larger space launch capability separate from the civilian efforts, selected the concept of using both stages of Titan II with various third and fourth stages, and later an array of zero stages. In very general terms, they retained the Agena for Thor and Atlas upper stages, and sought a new larger and simpler third stage, which was called Transtage. Agena was, however, regularly used on both Thor and Titan lower stages, and was often favored because it was a pump fed unit and was much lighter. The desirability of low chamber pressure, ablative chambers and a pressurized propellant feed system had been demonstrated in Vanguard, Hydra, and Ablestar, which led to the selection (for Titan) of the Transtage configuration using two chambers similar to, but larger than Ablestar. A variation of this became known as the Improved Delta stage, and was used extensively on Thor -so much so that the vehicle became known as Thor/Delta, and in some cases, only as Delta, or later as Thrust Augmented Delta, Long Tank Delta, Delta - PAM, and similar versions.

The resulting (improved) Delta launch vehicle underwent a bewildering succession of changes in configuration and performance (some incremental and some extensive) which is continuing to this day. All three stages have undergone upratings and replacements with more powerful engines, and there has been a succession of increasing numbers and sizes of strap-on solid propellant rocket zero stages. Aerojet's original ablative Delta engine had a radiation cooled nozzle exit section, burned N2O4/Aerozine-50 and produced a vacuum thrust of 9000 lbf. This unit also underwent several modifications, ending up with a thrust of 10,200 lbf, and a variety of tank sizes. A throttling (variable thrust) version was developed for a Navy application, and the Delta family could be generally distinguished from Transtage engines by the fact that the Delta gimbal point was at the head end (above the valve assembly), while the Transtage engines (in addition to being smaller) gimbaled at the plane of the nozzle throat.

In the early 1970s the expendable launch business was going quite well technically, but changes to reduce cost and increase payload were examined continuously. A series of Delta Alternative Studies was made, some being funded by NASA, but mostly in-house. One such study by Aerojet explored the concept of using a single barrel of the LR-87 engine. This would be used to improve Delta performance, but sufficient development funds were not available and the shuttle was receiving all the emphasis, so the idea died.
Another of these studies involved the use of liquid boosters in place of the nine solid boosters then attached to the Thor as a zero stage. The concept was to use either storable or LOX/RP-1 single LR-87 thrust chambers, pressure fed from filament wound tanks. The tanks would use thin metal liners to ensure freedom from leakage - as is still required today for high pressure, light weight filament wound vessels. Aerojet's solid rocket division had done some very encouraging work on high performance solid rocket engine chambers of this same type, and the prices were very attractive. The Aerojet team: Wally Dinsmore, Bob Frew, Lynn Meland. Howard Williams, and Chris Harlambakis from our Los Angeles office had many meetings and technical discussions at Huntington Beach. Two were memorable. The first was in October 1987 when their hosts kept slipping out of the meeting to check their stocks - this was the day the stock market fell drastically. No decisions were made that day. The second meeting almost didn't start. When the Aerojet contingent arrived, there was no one in the building because of an earthquake. The meeting finally got going, and just as Wally was to give his punch line cost and schedule estimate, the building shook with a severe aftershock and plaster dust fell from the ceiling. There were no decisions made that day either. This concept, like the others, couldn't get over the hurdle of the initial development cost - and probably the image of starting up a major new booster program.

Some additional insights into the early part of NASA's long succession of changes are provided by Dan Dembrow, who worked for NASA on the Delta Program at Goddard Space Flight Center:

The Delta launch vehicle was always viewed as an interim launch vehicle in NASA's stable, and was used to retain an intermediate size orbital payload capability until larger launch vehicles were developed. The Delta Project Manager at NASA, Bill Schindler, had a knack for keeping the program alive. He would participate in the launch vehicle assignment process, and when he found that Headquarters was about to kill the Delta on the grounds that a new mission exceeded its capabilities, he would assure them that Delta could do the job. Then he would challenge his technical staff, and often with the aid of the prime contractor, McDonnell Douglas, and our Goddard technical staff, to figure out a way to extend the flight envelope. This process occurred many times, and it was extremely difficult as production launch vehicles are not meant to undergo such constant changes in design. Delta payloads grew from about 200 lbm in low earth orbit in 1960 to about 4100 lbm in geosynchronous orbit by 1990.

In the 1960s, Goddard contracted directly for many of the components (including the second stage from Aerojet), and supplied them to McDonnell Douglas as government furnished equipment (GFE). During this period, Schindler oversold what Delta could do, and each successive payload had us in a quandary. Aerojet was always ready to test changes that would increase the second stage total impulse, and did so frequently. In one such case, a special test was required, and Aerojet went to a lot of trouble to upgrade the test stand for display to NASA management. Among the improvements was a new fail-safe system, and they even went to the point of repainting the bay. At the scheduled date the NASA managers had a schedule conflict, so I was the only NASA observer.

The test took place as scheduled, the instrumentation performed flawlessly, the test values were all nominal, and suddenly the test ended unexpectedly in mid-duration. Nothing within the control room showed anything wrong. When we went out into the test bay, the newly painted walls were on fire. The heat from the fire must have damaged some wiring in the fail-safe system, which promptly terminated the firing.

Aerojet did not realize the critical nature of the test, and this failure almost inadvertently killed the Delta program. Fortunately I was the only NASA observer, and a re-test was successfully conducted within a few days, so Schindler by that time was able to persuade NASA management that all was well. The critical decision was made to fly the mission on Delta.

The customary tug-of-war between the overall vehicle contractor and component system supplier over who should supply how much of the product, began as might be expected. As long as the project remained in Azusa, we had been able to keep the costs below anything Douglas could match. However, at the time when it was to be moved to Sacramento, management required the use of the "new" Commitment Proposal Estimating System which resulted in higher estimated costs. Also, the Douglas Huntington Beach plant was extremely low on work. As a final touch, Aerojet solved the problem by arrogantly sticking to the much higher price to our customer (Douglas). They promptly responded by replacing our system with a similar unit from TRW (surplus LM units) that unfortunately had slightly lower performance. Most of the production for NASA in that time period continued with this TRW engine. Douglas also had kept wearing away Aerojet's share of the project so that we went from supplying the complete stage to the present status of supplying only the thrust chamber assembly and propellant feed lines.

After a painful hiatus, we began work in 1975 on a complete "Fat Delta" stage for the Japanese N-II launch vehicle. At this point we received Air Force funding for performance improvement, one element of which was called the Delta I.T.I.P. (Improved Transtage Injector Program), and used an improved curved face, baffled injector. A still higher performance unit was developed, using a flat faced, baffled injector, and these were used in a two barrel configuration for a classified program. Their excellent performance made this chamber configuration popular, and was the basis for a long but relatively modest rate.

The Delta 3 introduced a new cryogenic upper stage in 1998. However the vehicle began tests just as the satellite market collapsed. It was flown only three times, two of the launches being failures. Meanwhile the most numerous and successful version, the Delta 7000 series, continued in service. The completely new-design Delta 4 was supposed to replace it in service, but the Delta 7000 remained not only lower cost but the most reliable launch vehicle produced in the United States. Nevertheless, in 2008, it was announced that production would be terminated and the Delta 4 would replace it.

The configuration of Delta vehicles was encoded for forty years as follows:

Optional letter after fourth character: An 'H' here indicated a 'Heavy' configuration. For Delta II, this was use of 46 inch diameter GEM solid strap-on motors in place of the standard Castor motors. For Delta IV, it indicated use of two strap-on Common Booster Cores (CBC) to supplement the CBC on the core stage.

Dash number: Payload fairing. For Delta II, this indicates the diameter of the fairing in feet. For Delta III or Delta IV, it indicates the length of the fairing in meters.

Delta A Nitric acid/UDMH propellant rocket stage. Able was only the first of many engine and application programs that flowed from the Vanguard experience base. These included Able, Ablestar, Delta, Fat Delta, the Japanese N II, and applications or offshoots such as Hydra, Saint (Satellite Intercept), and other classified programs.

Able-Star Nitric acid/UDMH propellant rocket stage. The Air Force requested increases in the propulsion system capabilities of the original Able upper stage design in an effort to meet their ever-expanding mission requirements. As a result, the stainless steel version of the basic Able engine was selected, and it was uprated to increase thrust 34.7 kN to 37.0 kN and to increase the duration 2-1/2 times (easily done with the stainless steel thrust chamber) - and this configuration was called Ablestar.

4300SS Thor SquadronThe USAF 392nd squadron trained the Royal Air Force with live launches. When the Thor missile was phased out in England, the 392nd become the 4300SS squadron with several 'all blue uniform' programs. This is a picture of the 4300SS squadron personnel taken sometime early 1964 or late 1963Credit: Larry Rhoads

Thor Burner 2Credit: US Air Force

Thor BurnerThor Burner - COSPAR 1965-021. Launched from Vandenberg AFB on St. Patrick's day of 1965. Standing at the launch mount are SGT Lipscomb (deceased as of 2002) and SGT Stovall (still living in Lompoc, CA as of 2002)..

Thor BurnerThor Burner - COSPAR 1965-003

Thor Burner 3Credit: US Air Force

Thor BlockhouseInside view of the Thor Burner I program block house looking at the launch control consoles (ca 1965).Credit: Larry Rhoads

Thor Burner Launch 2Pictures of the second Burner I launch (believed to be from launch pad 6). The first Burner I bird had a black second stage. This was an orbital launch with a permanent orbit, no recovery. I guess the thing is now space junk. A fully successful launch with all objectives met. Launch date was Thursday, March 17, 1965 sometime between 2100hrs and 2105hrs Vandenberg time. The night launch time of 2100 hrs with a 5 minute launch window was the same for both the first and second Burner I launches. Credit: Larry Rhoads

Thor Burner Launch 2Pictures of the second Burner I launch (believed to be from launch pad 6). The first Burner I bird had a black second stage. This was an orbital launch with a permanent orbit, no recovery. I guess the thing is now space junk. A fully successful launch with all objectives met. Launch date was Thursday, March 17, 1965 sometime between 2100hrs and 2105hrs Vandenberg time. The night launch time of 2100 hrs with a 5 minute launch window was the same for both the first and second Burner I launches. Credit: Larry Rhoads

Thor-Agena AThor Booster with an Agena satellite vehicle as the second stage with the launch name of DISCOVERER XIII. It was successfully launched from Vandenberg AFB on August 10th 1960. The Agena satellite orbited the earth and then was ejected from orbit and retrieved in the Pacific Ocean north of Hawaii the next day August 11, 1960 by a waiting Navy pickup ship. Credit: Larry Rhoads

Thor-Agena AThor Booster with an Agena satellite vehicle as the second stage with the launch name of DISCOVERER XIII. It was successfully launched from Vandenberg AFB on August 10th 1960. The Agena satellite orbited the earth and then was ejected from orbit and retrieved in the Pacific Ocean north of Hawaii the next day August 11, 1960 by a waiting Navy pickup ship. Credit: Larry Rhoads

Thor-Agena AThor booster built by Douglas Aircraft Company with a second stage Agena satellite vehicle built by Lockheed Aircraft Corp. This was a Douglas/Lockheed personnel launch operation from one of the Douglas launch complexes. Early to mid 1959.Credit: Larry Rhoads